One of the most important features of crown ethers is that they can form stable complexes with various metal salts, ammonium salts, and organic cation compounds. Using this property, various salts can be dissolved in organic solvents.
Crown ether can chelate cations into the ring, and at the same time, it can form complexes due to the outwardly facing organic genes, and it can also be soluble in non-polar organic solvents. At this time, the unsolvated anion exists in the solvent in the form of a naked anion, so it is extremely active. Crown ethers can dissolve alkali metals and organic alkali metal compounds in organic solvents.
Therefore, it has found a wide range of applications in organic synthesis, optical resolution, heavy metal chelation, separation, analysis, and physiologically active medicine and biochemistry. For example, as a phase transfer catalyst, many reactions that are difficult to react or even do not occur under traditional conditions can be carried out smoothly. This type of reaction has a fast rate, simple conditions, convenient operation and high yield.
For example, the yield of the condensation reaction of benzoin in an aqueous solution is extremely low. If 7% of crown ether is added to the aqueous solution, a 78% yield of benzoin can be obtained.
This reaction can also be carried out in benzene (or acetonitrile). Although potassium cyanide is insoluble in benzene, if 18-crown-6 is added, not only the reaction can occur, but the yield can be as high as 95%.